Field of the Invention
This invention relates to lithography, and more particularly, to a fly's eye mirror with optical elements rotationally aligned along two axes.
Description of Related Art
Extreme ultraviolet (EUV) lithography is a known semiconductor manufacturing technology that enables semiconductor wafers with extremely small feature sizes to be fabricated. In a typical EUV lithography tool, an EUV light source is generated from a plasma, such as either a Laser Produced Plasma (LPP) or a Discharge Produced Plasma (DPP). In either case, the EUV light is reflected off a mirror surface and into an illumination unit, which effectively acts as a condenser that collects and uniformly focuses the light onto a reticle. Projection optics then project the image defined by the reticle onto a light-sensitive photoresist material formed on a semiconductor substrate to be patterned. In a series of subsequent chemical and/or etching steps, the pattern defined by the reticle is formed on the substrate under the patterned photoresist. By repeating the above process multiple times, the complex circuitry of semiconductor wafer may be created on the substrate.
The illumination unit typically includes a pair of reflective fly's eye mirrors. Each fly's eye includes a plurality of faceted mirror surfaces arranged in an M×N array. During operation, the radiation from the light source is directed using a collimator onto the mirror surfaces of the first fly's eye. Each of the mirror surfaces reflects a portion of the light onto a corresponding mirror surface on the second fly's eye array. Each of the second fly's eye mirror surfaces is positioned in a pupil plane of a condenser, which condenses the reflected light onto the reticle. With this arrangement, the image field of each mirrored surface of the first fly's eye overlaps at the reticle to form a substantially uniform irradiance pattern.
With both the first and second fly's eye arrays, each of the faceted mirror surfaces should ideally be rotationally or tilt aligned along orthogonal axes. Current fly's eye fabrication techniques, however, result in rotational or tilt alignment only along one axis, but not in orthogonal axes. A conventional fly's eye mirror is fabricated by machining a plurality of monolithic blocks, each with a number of faceted mirror surfaces linearly arranged along a first axis. The monolithic blocks are then mounted, side-by-side, onto a base plate. With this arrangement, the faceted mirror surfaces tend to be in substantial rotational alignment about the orthogonal second axis, but not the first axis since the lineal length of each of the blocks in the first axis is greater than the lineal length of the block in the, orthogonal, second axis. As a result, rotational or tilt mis-alignment alignment of each of the blocks with a base plate will be greater about the first axis and smaller about the second axis as a result of the non-flatness of the mating surfaces of the block and baseplate.